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device-control-block.md

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The Device Control Bloc (dcb)

The DCB includes system registers that are common to the entire device and are not specific to any given hart.

Memory map

Offset Name Width Type Reset Description
0x0000 hartidmax 32b ro 0x00000NNN The highest value for hart ID in the device.
0x0004 vendorid 32b ro Vendor ID.
0x0008 archid 32b ro Architecture ID.
0x000C impid 32b ro Implementation ID.
0x0010 Reserved.
0x0020 dcsr Debug CSR.
0x0028 dpc Debug PC.
0x0030 Reserved.
0x0100 harts All harts interrupts.

The highest hart ID

For multi-hart devices, reading this register returns the highest numerical value used as hart ID in the device. Single-hart devices must return 0.

All harts interrupts

For multi-hart devices, these registers allow one hart to pend interrupts to any other hart, and possibly to temporarily adjust the priority thresholds, to handle synchronization issues like priority inversion.

Use case must be further investigated.

For single-hart devices, this area is reserved.

Offset Name Width Type Reset Description
0x0000 hartid 32b w 0x00000000 Access key.
0x0004 pendnum 32b w Hart interrupt pending bits.
0x0008 prioth 32b rw 0x00000000 Hart priority threshold.
0x000C Reserved.

These registers have one additional bit of state. To prevent inadvertent interrupt pendings, all writes to this area (pendnum and prioth) must be preceded by an unlock operation to the hartid register. The value (0x51F15000 + (Hart ID)) must be written to the hartid register to set the hart id and the state bit before any write access to pendings and prioth. The state bit is cleared at reset, and after any write to pendnum or prioth registers.

The pendnum register is write only and allows access to the hart identified by the write to hartid. Writing a small integer value pends the interrupt with the given number. The pendnum register must have enough bits to represent any interrupt number (at most 10).

The prioth register is read/write and allows access to the hart identified by the write to hartid. It must have enough bits to represent any interrupt priority.

Warning: The key mechanism has synchronisation problems in case multiple harts access it simultaneously. Implementations can choose to allow access only from hart 0.

RISC-V compatibility CSRs

The RISC-V Volume I, Chapter 2.8, mandates for the rdtime instruction. This can be retrieved from dcb.rtclock.counter.

Other RISC-V registers from RISC-V Volume II:

  • mvendorid
  • marchid
  • mimpid

Other registers that might need attention:

  • required for the debug module (are these per-hart or per-device?):
    • dcsr
    • dpc
  • optional for the debug module:
    • dscratch0
    • dscratch1